Method of making filamentary electromagnetic transducers



UCERS p i 1954 F. G. BUHRENDORF METHOD OF MAKING FILAMENTARY ELECTROMAGNETIC TRANSD 2 Sheets-Sheet 1 Original Filed Dec. 22, 1951 FIG. 2

//v l/EN TOP E 6. BUHPENDORF By W ATTORNEY April 27, 1954 F. G. BUHRENDORF 2,676,392

METHOD OF MAKING FILAMENTARY ELECTROMAGNETIC TRANSDUCERS Original Filed Dec. 22, 1951 2 Sheets-Sheet 2 FIG. 6 FIG. 7 /0 MIME/V701? E G. BUHRE/VDORF A T TOPNEV Patented Apr. 27, 1954 METHOD OE MAKING FILAMENTARY ELEQTRQMAGNETIC TRANSDUCERS Frederick G. Buhrendorf, Westfield, N. J.,' assignor' to Bell Telephone LaboratorieSi Incorporated, New York, N Y., a corporation of New York O ina a t n D e r 22, 1, S r

No. 252.959- Divided and'this application De tember t l9 2,.S ia 24.389

zfllaims.

This invention relates to magnetic heads for use in recording and reproduction, and more particularly to the method of making filamentary type magnetic heads of the kind shown and described in my copending application Serial No. 5 ance with the present invention; I 262,950, filed December 22, 1951, of which the Fig. 6 is a plan view of an embodiment of the present application is a division. present invention in which the transformer core The object of this invention is to provide a high is a bar element; frequency magnetic recording and reproducing Fig. '7 is a sectional view taken on the line 7'! head which is economical tomanufacture and of Fig.6 which is efiicient in operation, particularly in the 'Fig. 81s a plan view of an embodiment of the recording and reproduction of microsecond elecpresent invention in which the transformer core trical pulses. is rectangular;

In accordance with this inventionv the above- Fig. 9 isa sectional view taken on the line 9--9 noted requirements are satisfactorily met by a of novel magnetic head which comprises a trans- Fig. 10 is a plan view of an embodiment of the former core carrying two windings. The primary present invention inwhich the transformer core winding, which is to be-connected to the assooiis triangular; and ated equipment, consists of a convenient number Fig. 11 is a sectional View taken on the line of turns of insulatednwire, such as enamel cov- I-l-H of Fig. 10; ered copper. wire, the number of turnsbeing de- Referring now to the accompanying drawings fined by the associated equipment and the struc- 2 is the core of the'magnetic head and is preftural limitations of the core. The secondary e a v m e of a h permeability, ow os winding is closed. and consists .of one or several magnetic material such as ferrite. The endless turns terminated in a slender. filament of cone secondary winding 4 consists of one turn as shown ducting material disposed in a groove in the in Figs. 1, 2, '6,- 8 and 10 or a plurality of turns as outer peripheral face of the core. Flux surround: shown in Figs. 3 and 4 of a low resistance material ing this filament, when current is induced in it, such as copper or silver, and is terminated in a is usedto record on the magnetic medium in slender filament of low resistance, conducting much the same manner as in conventional air, material 6 which is flush with the surface of the gap type heads. In reproduction, current induced O t r edge 0 i ofthe Core: It 16150 be n in this filament generates a flux .in-the core and, stood that an endless secondary winding conby virtue of the transformeraction, a voltage is sisting of a plurality of turns may also be emdelivered to the associated equipment by the ployed with each of the transformer cores shown larger primary winding. in Figs. 6, 8 and 10. The primary winding 8 An important feature of this invention resides. c mpr e a plurality f u ns of i su ated 10w in the structure of the secondary winding and resistance W u as'enamel Covered op the method of disposing said Winding directly on Wire, Wound in such manner as to be coupled the surface of the core when said core is made of cl y to th Core the Secondary Winding a reasonably non-conductive material such as The'number of turns in this winding is defined ferrite. 40 by the associated equipment to which it is to be The nature of the invention and its distinguishc n e d b means of t terminal Wires and ing features and advantages will be more clearly l2, and 7 5 ctl limitations of h C understood from thefollowing detailed descriph f y 35 may be Obtained with tion and the accompanying drawings in which: P m accgrdance l the Present invention Fig 1 is a plan View of an embodiment of the in which the filament lies at an angle with respect present invention in which th tran orm core ggr fi n igr lig ifii rrf ii zf ci se zfid z n gl i g :1 1tgrmdal and the secondary winding is a single mounting the filament sh 0111 d be located parallel to the axis of the core as shown in each of the Fig. 2 1s a View in perspectiveof the secondary embodiments of the present invention wmfimg m E For optimum results the filament should lie in l 3 1S plan VleW of emb d nt 9 th a plane parallel to the surface "of therecord pre e inventionin which n me co e medium and normal to the direction of the rela- 5 toroidal an the y-winding cow/ 1. 9f tive motion of the recording hea d with respect in to the record medium;

a plurality of turns;

2 Fig. i is a view'in perspective of the secondary winding shown in Fig. 3

Fig. 5 is a view in'perspective showing the actual dimensions of a magnetic head in accordeese The particular embodiments of this invention as shown in Figs. 1 to 11 are intended for use with a right circular cylindrical record medium revolving about its axis. The recording head is placed so that the filament clears the surface of the cylinder by 0.002 inch and the axis of the core of each of the embodiments, except the one shown in Figs. 6 and 7, is parallel to the axis of the cylinder. The axis of the core of the embodiment shown in Figs. 6 and '7 is normal to the axis of the cylinder.

In the preparation of the magnetic head, the secondary winding may be applied directly to the surface of the core 2. It has been found that the following procedure will provide a satisfactory secondary winding. A fine groove i4 substantially 0.003 inch deep and 0.002 inch wide and substantially parallel to the axis of the head is cut in the edge of the core. The entire core is then coated, by means of a process such as the Brashears silvering process, with a thin coating of metallic silver and given an electrodeposited copper flash. The entire core will have, at this point of the process, a cop-per coating approximately 0.0001 inch thick. The core is then masked with a stop-off lacquer, such as nitrocellulose lacquer, to form an electrical path around the core such as the single turn path shown in Fig. 2 or the path consisting of a plurality of turns shown in Fig. 4. A copper or silver plating 0.003 inch thick is then applied to the core. After the plating operation the lacquer is removed with a suitable solvent, such as acetone, and the entire core is etched until the surfaces previously masked are free of applied metal. The final operation is to lap off the surplus metal on either side of the groove Hi, which is completely filled with copper, or silver, so that only a slender filament 3 will remain to complete the winding at that point.

The filament 5 should be flush with the surface of the outer edge of the core. It should not be underflush and should not extend beyond the outer edge of the core. The width of the filament should be as narrow as practicable. Widening oi the filament widens the recorded spot or area on the record medium and also reduces the efficiency of the recording head.

The present invention eliminates the high concentration of flux which occurs at the pole tips in magnetic heads having conventional air-gaps and, therefore, permits the use of ferrite or other similar materials which, although saturating easily, have other characteristics which are most desirable for the recording and reproduction of high frequencies. For example, ferrite has extremely high resistivity and as a result the eddy currents in the present invention are very low. This important feature is desirable for efficient operation in the recording and reproduction of microsecond electrical pulses.

It has been found that the embodiment of the present invention in which the secondary winding is a single turn, as shown in Figs. 1, 2, 6, 8 and 10, will provide satisfactory results, particularly at the higher frequencies. The embodiment of the present invention in which the secondary winding consists of a plurality of turns and the primary is wound directly over the secondary, as shown in Figs. 3 and 4, may be used to greater advantage in those applications where closer cou-- pling between the primary and secondary windings may be desirable.

The conductor of the secondary winding of each of the embodiments of the present invention should have as large a cross-sectionaspossible,

the area of said cross-section being limited only by the distributed capacity in a turn of the winding. It has been found that the optimum average width of the conductor of the secondary winding is equal to substantially one-tenth the average length of the magnetic path in the core.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

i. The method of making an electromagnetic transducer including a magnetic core and a conductive winding disposed directly thereon, said winding having a substantially uniform predetermined thickness and including a segment of predetermined filamentary width, which comprises disposing in the outer peripheral face of said core a groove having a depth equal to said predetermined winding thickness and a width equal to said predetermined segment width, depositing a relatively thin first coating of low resistance metal on the surface of said core, aiiixing a material to the surface of said coated core in such manner as to form an unmasked surface pattern of said conductive winding, plating said core to add a second coating of low resistance metal to the unmasked portion of said coated core which includes said groove, said second coating having a thickness equal to said predetermined winding thickness, removing said masking material, and etching the entire surface said core to remove said first thin coating of low resistance metal from that portion of the core surface which had been masked.

2. The method of making an electromagnetic transducer including a transformer core and a closed conductive winding disposed directly thereon, said winding having a substantially uniform predetermined thickness and including a segment of predetermined filamentary width, which comprises disposing in the outer peripheral face of said core a groove having a depth equal to said predetermined winding thickness and a width equal to said predetermined segment width, depositing a relatively thin first coating of low resistance metal on the-surface of said core, affixing a masking material to the surface of said coated core in such manner as to form an unmasked surface pattern of said conductive winding, plating said core to add a second coating of low resistance metal to the unmasked portion of said coated core which includes said groove, said second coating having a thickness equal to said predetermined winding thickness, removing said masking material, etching the entire surface of said core to remove said first thin coating of low resistance metal from that portion of the core surface which hadbeen masked, removing surplus metal from either side of said groove to define said predetermined filamentary width, and disposing an open winding on said core.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,647,474 Seymour Nov. 1, 192'? 1,767,715 Stoekle June 24, 1930 2, 6 11 Arlt -1 Jan. 5, 193? 2,230,913 Schuller Feb. 4, 1941 2,538,405 Zenner Jan. 1 1951 2,616,994 Luhn Nov. 4, 1952 

